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This post is to share some tips at improving the efficiency of turboshaft engines, driving propeller blades (airplanes) or rotor blades (helicopters). The concepts should apply equally to electric rotors driving propellers or heli-blades. I'm using 1.7.3, the electric rotors, turboshaft engines, and propeller/heli blades are new to stock KSP, so in later versions some of this info might become outdated.
It's easy to focus on either the Turboshaft Engine, or the Prop/Heli blades when troubleshooting 'why doesn't my vessel work'. The reality is you need to look at both elements to make a functioning prop-airplane or helicopter. The engine produces torque, which spins a shaft. This is important, torque produces zero thrust. Add all the torque you want, it will not produce any thrust- until it is coupled to a lifting device, such as a propeller blade or heli blade.
On the other hand, the prop/heli blade also produces no thrust unless it is moving. You can angle the blades any way you want, they will produce no thrust unless they are moving. The movement is provided by the Turboshaft Engine (or electric rotor).
As of 1.7.3, the stock engines can rotate at a maximum rate of 460 rpm, which is evidently a Unity limitation. Adding torque will increase rotational speed (rpm) up to that limit of 460 rpm. What happens if you are already at 460 rpm, and increase torque? That's an important question. What happens (in 1.7.3) is you increase the Fuel Flow (or EC draw), but do not get any increase in rpm. Beyond 460 rpm, adding torque is simply wasting fuel.
In the following examples, we're going to experiment with turboshaft engines. There are two variables we are going to adjust, and unfortunately they are on two different parts. One is the Torque of the Turboshaft Engine, the other is the Authority Limiter of the prop/heli blades. To make things easy, I like to assign these values to action groups. I assign Engine Torque to the Main Throttle, and the Blade Authority Limiter I assign to Translate Forward/Back (usually 'H' and 'N').
Okay, lets get started. Here's a turboshaft engine we're going to play with. It's in the 'propeller' configuration, but the idea is the same for helicopters.
Ok, we have an engine built on a stand, let's play with it to see how Torque, Blade Authority, RPM, Fuel Flow, and Thrust are related.
In the next spoiler window, I'll play with Torque and Blade Authority Limit, to see what happens to rpm and Fuel Flow. Keep in mind that Thrust is only dependent on rpm and blade angle. If my rpm and blade angle stay constant, but my Fuel Flow increases, I have not increased my thrust. I'm just wasting fuel at that point. So, one goal is to find the minimum Fuel Flow which will maintain a specific rpm at a specific blade angle. Here goes.
In the next spoiler window, I'll demonstrate an actual aircraft, making adjustments to torque (and Prop Authority) to reduce fuel burn. Ideally I'd use a single-engine airplane to keep things simple. However, counter-acting the torque effects of a single-engine are difficult to design for, and fly efficiently. It's easier to just make a plane with two counter-rotating engines and propellers. Then, it flies quite easily just like a jet, with no nasty torque effects.
The takeaway here is that to get the most out of the turboshaft engines and propeller or helicopter blades, you need to look at more than just the engine or just the blades. Both elements need to be adjusted for optimum performance. Ideally, you need to be able to adjust both Torque and Blade Authority Limit in flight, to adjust for varying conditions. Regarding efficiency, the big takeaway is that adding Torque beyond what is required to maintain rpm is just wasting fuel, and lots of it.
As of 1.7.3, I believe the concept is the same for electric rotors driving propellers or heli blades- adding torque beyond what is required is only wasting EC.

This is a stock replica of the GP7000 Turbofan developed by Engine Alliance, a joint venture between General Electric and Pratt & Whitney. This replica’s central feature is definitely its spinning turbine, which is functionally a turboshaft as it is powered by two Juno’s set to very low power. This method of spinning the turbine means that the turbine actually throttles with the actual engines, which is obviously super cool. This engine uses 3 Wheesley’s which in total provide similar thrust to the real thing. The one downside of this engine is that you will need to place intakes in a seperate location on your craft, as there is no way of incorporating intakes into the engine itself.
Download Link: https://kerbalx.com/Kronus_Aerospace/GP7000
Part Count: 113
Engine Mass: 9.97 tonnes
Engine Thrust: 330 kN

The Kraken Killer
Somebody asked me if I could create a "futuristic" looking helicopter and this is my response! Drawing inspiration from all across the board, including the Halo Pelican, the Fallout Vertibird, the C&C Orca, the Aerospatiale SA-2 and so many more. Though it may look different, the VK-02 is a traditional Turboshaft vehicle. Pumping out about 270kN per rotor and with a top speed of around 115 m/s. The Kraken Killer is easily the most stable and fastest Kermansky Helicopter to date. Even the greenest beginner should be able to pilot this craft with little trouble!
How to Operate:
1. Releases Rotors
2. Toggle VTOL engines
3. Toggle all Afterburners (craft will lift off if only VTOL is activated)
4. Toggle 2 of 4 Forward movement engines
5. Toggle the other 2 of 4 Forward engines

Kermansky KM-60
KerbalX.craft file
Borrowing a little bit from the looks of the Sikorsky MH-60, the KM-60 is the culmination of hours of trial and error and hours of taking apart other people's helicopters! Able to lift a total of about 60 tons. She uses 4 panther blowers to spin the 4 bladed rotor and a single panther with full gimbal enabled to counter the torque-spin and to help with unwanted roll at speed. Limited to about 40-50 m/s she isn't too quick but is user friendly.
Controls:
1. Release Turboshaft
2. Toggle Main Engine
3. Toggle Wet Mode (lift off)
4. Toggle counter-steer engine